1. Field of Invention
The present invention relates to a sample concentrator system. More particularly, the present invention relates to an online sample concentrator system based on solvent evaporation for use in a chromatography system.
2. Description of the Related Art
Preconcentration in a chromatography system, performed prior to detection of the analytes, typically is performed by concentrating the analytes in a column that is preferably selective to the analytes of interest and then subsequently eluting the trapped or concentrated species for further analysis. When analyte concentrations are too low to produce an adequate detector response a variety of solutions are implemented. Traditional solutions to this problem include prior sample workup to increase analyte concentration, large volume injection with or without preconcentration on a solid phase, adoption of a more sensitive detector, or a change to a more sensitive detection approach that may involve either pre- or post-column derivatization. Preconcentration methods using a solid phase are more prevalent in ion chromatography. For example when pursuing trace level analysis of anions in ultra pure water (UPW) samples, preconcentration is typically used to concentrate the anions onto a concentrator column. In addition to anions, the approach also concentrates the dissolved carbon dioxide which manifests as carbonate during analysis. Depending on the concentration, the presence of the carbonate peak may interfere with quantitation of other anions such as sulfate. For UPW samples it would be useful to have a method that only concentrates the anions without the dissolved carbon dioxide. When pursuing trace analysis with environmental samples such as drinking water the detection of trace ions in the presence of matrix ions is an issue. Concentrating the trace ions also typically concentrates the matrix ions. It would be useful to have a selective concentration mechanism.
Other means of improving sensitivity is presently pursued by preferentially eliminating the solvent using solvent condensation or evaporation techniques. Nitrogen or inert gas assisted evaporation, distillation, rotary evaporation, evaporation with tube heaters, lypholization and the like are different approaches used in preferentially removing the solvent. These are all typically offline methods that are implemented using bench top equipment and as such are not amenable for use with chromatographic equipment or inline chromatography.
In liquid chromatography there are at least two detection approaches that incorporate the concept of solvent removal before detection. One involves the interface between liquid chromatography and mass spectrometry (LC-MS 1, 2). Another application area is in evaporative light scattering detection (ELSD,3). In both of these approaches, the detector is mass-sensitive, rather than concentration sensitive and detection is accomplished in the gas phase. Additionally the rationale for removing the solvent was to improve compatibility with the detector. For example the interfaces such as thermospray, electrospray, atmospheric pressure chemical ionization and ionspray were all designed to reduce the solvent being transmitted to the mass spectrometer. The above detectors are all mass sensitive detectors.
In recent years, technological developments have permitted commercialization of capillary and intermediate scale chromatographic equipment. These new generations of low cell-volume concentration sensitive detectors that for example measure optical absorbance, electrical conductance or fluorescence display the same (or even better) concentration limits of detection (LODs) compared to their older larger cell volume counterparts. Nevertheless, the majority of liquid chromatography practice still centers on standard bore (4.0-4.6 mm i.d.) columns with a typical eluent flow rate of the order of 1 mL/min. In the vast majority of cases, if the column effluent from such a system is split and only 10% of the original flow is sent to a low cell-volume detector, there will be no apparent deterioration of sensitivity compared to the entire effluent going through a standard, larger cell volume detector. It would useful if the sensitivity when operating with concentration sensitive detectors can be improved.
The patent literature includes systems that use a batch mode of operation for concentrating the analyte stream. These approaches do not preserve the separation and involve multiple steps and transfer processes that can potentially hinder the recovery and quantitation of analytes of interest.
For example, U.S. Pat. No. 4,055,987 describes an interface between a liquid chromatograph and a mass spectrometer. The interface comprised of a thin ribbon which received the effluent from the liquid chromatography column and the ribbon is passed into several vacuum locks to remove the residual solvent. The residual solute is flash vaporized into the ionization chamber of the mass spectrometer.
U.S. Pat. No. 4,465,554 describes an apparatus for evaporating liquid fractions by directing a hot steam of non reacting gas directly on the surface of the sample fraction collected to aid evaporation. This approach is an offline approach and by collecting the fractions, the separation is not preserved.
U.S. Pat. No. 4,604,363 describes an automatic evaporator system performing evaporation and concentration coupled with solvent exchange. The samples are delivered to a temperature and pressure controlled evaporation chamber. This again is a batch mode device.
U.S. Pat. No. 4,867,947 describes a moving belt interface that strips away the eluent continuously while leaving behind a residue for analysis by a mass spectrometer. A probe tip was also disclosed that sampled the residue by direct ionization. The peak shapes are not preserved, and this method is not well suited for post analytical steps.
U.S. Pat. No. 5,897,838 describes an apparatus for concentration or evaporation of aqueous solutions. The approach involved a combination of vacuum and directing an air stream on the sample that is residing in a vial in a multiplate well.
U.S. Pat. No. 5,100,623 describes a temperature controlled water bath for the function of evaporation. The apparatus also incorporated means of adding solvents to reconstitute the sample.
U.S. Pat. No. 6,146,595 describes a closed positive evaporation system for evaporating samples prior to analysis. This approach is not continuous.
U.S. Pat. No. 6,620,620 describes batch mode apparatus that facilitated controlled evaporation of a liquid. The apparatus includes a deposition surface plate on which the sample to be analyzed is deposited in a drop by drop fashion.
U.S. Pat. No. 6,656,361 discloses a membrane assisted evaporation process to dry brine solutions. The purpose of this work was to replace drilled oil from salt caverns with salt rather than diluted salt solution. Therefore there was a need to concentrate or evaporate the water from the brine solution.
U.S. application 20040203175 describes an apparatus for concentrating one or more analytes in a flowing liquid stream. The solvent components are evaporated by flowing the sample stream through a transfer tube that is heated and evaporation was accomplished on hanging droplets of the flowing samples. Partial evaporation is accomplished by operating at a temperature below the boiling point of the solvent. The drops are accumulated in a collection device that is also heated to effect further evaporation.
Bishop and Mitra (J. Chromatogr. A 1046 (2004) 11-17 and J. Pharmaceutical and Biomedical Analysis 37, 2005, 81-86) describes an inline preconcentration method by evaporating the solvent from a sample stream in a membrane based evaporator module prior to injection onto a HPLC system for analysis. The hollow fiber based evaporator module was fitted with five strands of hollow fiber membranes comprising of polypropylene substrate that was coated with siloxane. Another device used a commercially available Nafion® membrane from Perma pure, Toms River, N.J. USA) and designed for gas phase drying applications. The second reference cited above describes the use of the Nafion® membrane-based module. A heat tape was used to heat the hollow fiber module. In the first cited publication the sample after evaporation was collected into a sample vial for further analysis. The above method was a pre injection sample pretreatment approach and suffered from the limitation of coping with possible contamination issues from the sample vials. The second publication describes a direct interface to the injection valve. The sample was delivered to the hollow fiber module using a pump. Both devices allowed evaporation of solvent from the sample stream thereby concentrating the sample stream.
In suppressed ion chromatography there is a need for a non chromatographic concentration method. Additionally, there is a need for a concentration apparatus that would enhance the sensitivity without significantly affecting chromatographic separation. Additionally in multidimensional separation schemes it would be useful to have an inline approach thereby avoiding any sample transfer related loss. It would also be useful to have a method that would concentrate analytes selectively.